The development of techniques associated with mobile devices and increase in demand therefor have brought about rapid increase in the demand for secondary batteries as energy sources, and accordingly, studies of the secondary batteries, which can meet various demands, are being on the rise.
Among the secondary batteries, demand for prismatic type secondary batteries and pouch-type lithium secondary batteries that can be applied to products such as mobile phones and the like due to their small thickness in terms of battery shape is high, and demand for lithium cobalt polymer secondary batteries with excellent energy density, discharge voltage, and safety and in terms of materials constituting the batteries is high.
In the case of the prismatic lithium secondary battery, it is advantageous in protecting an electrode assembly from external impact and easy to a liquid injection process. However, since its shape is fixed, it is difficult to reduce the volume thereof. Thus, in the case of electrical products using this as a power source, there are limitations in their design. Also, in terms of safety, since an effect of venting gas or liquid is not smooth, internal heat and gas are accumulated. Thus, the risk of explosion may be high, and the time required to cause cell degradation may be short due to overheating because the internal heat is not effectively released.
On the other hand, in the case of the pouch-type lithium secondary battery, it has advantages in that it is particularly suitable for production of a thin cell because there are no limitations in shape and size, assembly by thermal fusion is easy, and it has high safety because the effect of venting gas or liquid is facilitated when abnormal behaviors occur. However, since it uses a thin soft laminate sheet (pouch) as a case, compared to the prismatic secondary battery, it has a disadvantage of having low stability to external impact and the like due to its weak physical and mechanical strength and low reliability of sealing.
In particular, in the case that high current flows inside of the pouch secondary battery in a short time by local crush such as exposure to high temperature, overcharging, external short, foreign substances in an electrode, nail penetration and the like, the electrode coated with an active material provides a heating source to generate heat. Accordingly, there are problems that reaction between an electrolyte and the electrode is accelerated as the battery temperature rapidly increases, thereby causing ignition of the battery, and also the secondary battery is swollen due to internal pressure of the battery increased by the gas generated by the reaction between the electrolyte and the electrode. Because this risk of explosion causes serious stability problem, it is the most fatal disadvantage of the lithium secondary battery.
Accordingly, a factor to be necessarily concerned when developing the pouch-type secondary battery is to secure stability to external impact. For this, a method for reinforcing strength of a battery by additionally forming a separate strength reinforcement layer on the surface of the conventional soft pouch case was suggested. However, in this case, because an extra strength reinforcement layer other than the pouch case should be additionally made, it has disadvantages of increase of battery size, volume and weight, and production cost of the battery.